a. Technical Field of the Invention
The present invention relates to a silencer having a noise control system employing the principle of active attenuation.
b. Prior Art Technology
There has already been proposed a noise control system employing the principle of active attenuation. (U.S. Pat. No. 4,527,282). This method is implemented in such a manner that vibrations from a driver or noise-killing speaker are introduced into a acoustically partially closed volume whose largest dimension is several times smaller than the wavelength of the highest frequency of the vibrations which are the object of noise cancellation and a microphone is disposed in said volume to stabilize the sound pressure in the neighborhood of the microphone, whereby the frequency range over which noise cancellation is effective is expanded.
A noise attenuation system of this type can be utilized for cancelling the noise of the machinery disposed within a machine chamber having an opening for heat dissipation. As a typical example of machinery installed in a machine chamber having an opening for heat dissipation is a refrigerator compressor.
The application of this concept of active attenuation of noise to a refrigerator is now described with reference to the schematic diagram presented in Fig. 23.
Disposed in the machine compartment 10 in the lowermost position at the back of a refrigerator is a compressor 20 which is a principal source of refrigerator noise. This machine compartment 10 is hermetically closed except at an opening or opening 17 available for dissipation of heat and water vapor due to defrosting and has a one-dimensional duct structure. That is to say, compared with the wavelength of compressor noise S to be reduced, the sectional dimension of the duct is made sufficiently small so that the compressor noise within the machine compartment 10 will be a one-dimensional plane progressive wave. The compressor noise S is detected by a microphone 35 disposed within the machine compartment 10 and far away from the opening 17. The compressor noise detected by the microphone 35, that is the detected sound M, is processed by a control circuit 40 with a transfer function G which, for example, has a finite impulse response filter (hereinafter referred to as FIR filter) which processes signals in the time domain as such, and, then, is fed to a speaker 50. The controlling sound A from this speaker 50 cancels the compressor noise which would otherwise emerge unattenuated from the machine compartment opening 17.
The transfer function G of the control circuit 40 is determined as follows.
First, the sound M detected by the microphone 35 can be expressed by means of the following equation (1) EQU M=S.times.G.sub.SM +A.times.G.sub.AM ( 1)
where S is the noise produced by the compressor 20, A is the controlling sound output of the speaker 50, G.sub.SM is the acoustic transfer function between the compressor and the microphone, and G.sub.AM is the acoustic transfer function between the speaker and the microphone.
Provided that a microphone 55 for the evaluation of noise-attenuating effect is installed at the machine compartment opening 17, the measured sound R at this evaluation microphone 55 can be expressed using the following equation (2) EQU R=S.times.G.sub.SR +A.times.G.sub.AR ( 2)
where G.sub.SR is the acoustic transfer function between the compressor and the opening and G.sub.AR is the acoustic transfer function between the speaker and the opening.
Since G is the transfer function between the microphone and the speaker, the following equation (3) holds. EQU A=M.times.G (3)
Now, in order that the compressor noise that would emerge from the opening 17 may be cancelled, the following equation should hold true. EQU R=0 (4)
From the above equations (1) through (4), the transfer function G for noise cancellation is expressed by the following equation (5). EQU G=G.sub.SR /(G.sub.SR .times.G.sub.AM -G.sub.SM .times.G.sub.AR) (5)
Dividing the denominator and numerator of the above equation (5) by G.sub.SM gives the following equation (6), provided that G.sub.MR is defined by equation (7). EQU G=G.sub.MR /(G.sub.MR .times.G.sub.AM -G.sub.AR) (6) EQU G.sub.MR =G.sub.SR /G.sub.SM ( 7)
By using these equations (6) and (7), the transfer function G necessary for the measured sound R to be 0 can be found by measuring the transfer function ratio G.sub.MR of G.sub.SR to G.sub.SM even if the compressor sound S is unknown. Thus, all that is necessary is to input the detected sound M to produce a measured sound R as a response while the compressor 20 is allowed to produce noise S.
By applying the transfer function G determined in the above manner to the control circuit 40, a controlling sound A corresponding to compressor noise S can be generated to thereby cancel the noise S at the machine compartment opening 17.
In applying the above active attenuation method, the schema of detecting compressor noise S by the microphone 35 presents the following problems.
The first problem is that the microphone 35 receives not only the noise S of compressor 20 but also the controlling sound A from the speaker 50. This can lead to positive acoustic feedback, or howling. This means that the output of the speaker 50 cannot be raised to a sufficiently high level to obtain the desired noise-reducing effect. This problem can be solved by interposing an echo canceller for prevention of howling in the control circuit 40 but the practice results in added cost.
Furthermore, in case a cooling fan for the compressor 20 is installed within the machine compartment 10, the microphone 35 picks up the noise from the fan as well, thus complicating the noise control system. Furthermore, there is a constant risk for the noise reduction system responding to external noise.